This invention relates to a method of forming a tappet body for use in a direct acting type valve operating mechanism in an internal combustion engine.
Recently aluminium alloy tappets (valve lifters) have been used in a direct acting type valve operating mechanism of DOHC type to provide a lighter valve mechanism, thereby increasing engine capability. Al alloy tappets have lower strength, rigidity and wear resistance compared with steel tappets, and wear resistant metal is applied to the surfaces which contact a rotary cam and an engine valve.
Such Al alloy tappets are illustrated in FIG. 6. A tappet "A" comprises a substantially circular upper wall 1; a substantially cylindrical skirt 2 which extends from the circumference of the upper wall 1; and a cylindrical shim receiving portion 3 which extends on the circumference of the upper wall 1 to constitute an Al alloy tappet body 4. With the shim receiving portion 4 is engaged a circular outer shim 5 made of wear resistant matel. The tappet body 4 is engaged with a rotary cam 6 via the outer shim 5, and a circular shim 8 made of wear resistant metal is engaged with a recess 7 in the middle of an inversed frustoconical thickened portion la on the lower surface of the upper wall 1. The tappet body 4 is engaged with the axial end of an engine valve 9 via the inner shim 8.
Conventionally, to form the tappet body 4, as shown in FIG. 7, there is provided a counter punch 113 the upper end of which fits the lower surface of the finished tappet body 4 at the bottom of a guide bore 112 of a die 111. On the counter punch 113, Al alloy cylindrical material 114 is placed, and a punch 115 the lower end of which fits the upper surface of the finished tappet body comes down, thereby forming a tappet by a single step of cold forging.
In the tappet for use in a direct acting type valve operating mechanism, the inner shim 8 directly contacts the engine valve 9, so that the recess 7 in which the inner shim 8 fits is repeatedly subjected to large compression stress. Accordingly, the thickened portion la around the recess 7 requires high strength and rigidity. However, when the recess 7 is integrally molded with the thickened portion 1a once, a relatively large flow "B" is formed in the thickened portion 1a around the recess 7 as shown in FIG. 8, so that metallic structure (crystal) around the recess 7 becomes more coarse, thereby providing poor strength and rigidity against compression load and sideward pressure. Further, the thickened portion la around the recess 7 mechanically formed becomes rough laminate structure, which is inconvenient against compression stress and sideward pressure. When rigidity around the recess 7 is low as above, the recess 7 is deformed to cause a play with the inner shim 8, thereby decreasing holding force to the inner shim 8 and increasing a gap to generate noise.
In the known method as above, the shim receiving portion 3 is too short compared with the skirt 2, so that the punch 115 contacts the material 114 and forming of the shim receiving portion 3 terminates when deformation of the material 14 begins. The material extruded by descent of the punch 115 all flows towards the skirt 2, and as shown in FIG. 9, at a branched portion of the skirt 2 and the shim receiving portion 3 in the upper wall 1, there is formed a boundary "E" between a flow-stopping portion "C" and a skirt-directing portion "D". Flow separation is liable to occur, thereby decreasing strength and, at worst, generating cracks. To prevent such flow separation, forward and backward extrusion without restricting the ends of the skirt 2 and the shim receiving portion 3 has been suggested, but the shim receiving portion 3 is too long, thereby increasing mechanical working cost and thus decreasing material efficiency or yield.
In view of the disadvantages in the prior art, the object of the present invention is to provide a method of forming a tappet body in an internal combustion engine in which metallic structure becomes condensed to increase strength, rigidity and yield by improving flow in the material to be formed during forging.